Method and apparatus for heating fluids



Jan. 7, 1964 J. D. NESBITT v 3,116,915

METHOD AND APPARATUS FOR HEATING FLUIDS Filed Jan. 6, 1961 2 Sheets-Sheet 1 U l- J INVENTOR JOHN D. NESBITT ATTORNEY Jan. 7, 1964 J. D. NESBITT 3,116,915.

METHOD AND APPARATUS FOR HEATING FLUIDS Filed Jan. 6, 1961 2 Sheets-Sheet 2 v INVENTOR JOHN D. NESBITT a dzx my AT TORNEY United States Patent 3,116,915 lvlETHQD AND APPARATUS FQR HEATING FLUHDS John D. Nesbitt, Toledo, @hio, assignor to Midland-Ross Corporation, Toledo, @rio, a corporation of @hio Filed lab. 6, 1951, Ser- No. 81,118 Claims. (Q1. 263-19) The present invention relates to a method and apparatus for heating fluids, and more specifically to the heating of liquids and gases in chambers. The invention is particularly concerned with the heating of fresh make-up air as is required industrially to replace air that is exhausted through spray booths, vent hoods, dust collectors, furnaces and the like. Likewise the invention may be used to reheat recirculated air or a mixture of recirculated air and fresh make-up air.

Conventional make-up air supply equipment usually includes one or more circulating fans, air filtering equipment, heatin equipment, and a sheet metal duct which encloses both the air stream and the above elements. It is also relatively common practice to heat liquid baths with submerged combustion means, and the present invention provides a novel improvement in this type of heating means.

The heating means heretofore predominantly utilized in make-up air heaters, have utilized a type of pre-mix burners commonly denoted as line burners. Such burners normally comprise elongated ported tubes to distribute pre-mix including baiiles for flame retention and stabilization in combination with mixing means adapted to deliver a homogeneous mixture of fuel and air to the elongated tubes. The elongated tubes are normally mounted within the chamber formed by the sheet metal duct work, and the mixing means are mounted on the outside of this chamber. Suitable piping means are provided to deliver pre-mix from the mixing means to the ported tubes. Ignition means, such as pilot burners, are normally provided to ignite the combustible mixture which issues from the tubes.

The application of line burner equipment in a confined duct system such as fresh make-up air heaters or recirculating air heaters inherently involves numerous objectionable and hazardous features which have heretofore been tolerated only because the art has lacked a more suitable combustion system. Some of these features are as follows:

(1) The elongated ported tube or pre-mix manifold portion of the line burner is made up from several sections jointed together, and these joints may leali pre-mix directly into the fluid stream being heated with considerable risk of confined explosions.

(2) Line burners are normally piloted at one end of the ported manifold with a flame protection device located at the opposite extreme of the ported manifold to insure positive ignition over the entire length of the burner manifold. This system can fail unsafe if after the initial ignition, failure occurs at an intermediate point on the ported tube. This frequently occurs by plugging of some of the pre-mix distribution orifices or ports or by manifold leakage or local high velocities which quench the flame.

(3) Pre-mix systems inherently have a limited degree of turn-down; therefore, in a typical line burner-duct assembly, a plurality of individually controlled burner sections must be provided so that individual burner sections can be turned on and off to obtain the effect of adequate turn-down. This practice inherently affects safety economy (because of the necessity of providing a plurality of valves, control device, etc.) and temperature distribu- :metal with refractory.

insets Patented Jan. '7, 1964 tion (because of varying firing pattern which results from some of the burners being alternately on and off).

(4) Use of pre-mix line burners often causes another severe temperature distribution problem because of the high temperature differential between the flame temperature and the temperature of the fluid being heated, and this is accentuated on turn-down for the reasons given above.

The present invention is also especially well suited to submerged combustion heating systems commonly used for heating various types of liquid baths such as water baths, pickling solutions, evaporating solutions and other applications wherein the liquid being heated is not adversely affected by direct contact with products of combustion. The prior art shows various types of nozzle mixing burner equipment for submerged combustion applications, but such disclosures have one feature in common, i.e. the point where combustion is initiated is below the liquid level of the bath being heated in order to avoid overheating of intermediate metal parts. The practice of firing below the bath level complicates burner construction, ignition and piloting problems to a very great extent e.g. long ignition, piloting flame protection, etc. extensions are necessary, and means for purging the liquid to be heated from the combustion products manifold is necessary for start-up. In cases where an external burner has been used it is necessary to protect the intermediate Such refractory is particularly vulnerable, being easily destroyed or damaged by physical and thermal shock, corrosion from the liquid being heated and attrition from flame impingement.

it is, therefore, an object of the present invention to provide a novel combustion system for heating a stream of air passing through an air supply duct, or supplying heat to a liquid bath by direct contact of the products of combustion with the liquid to be heated.

It is a further object of the invention to eliminate the hazards involved in transporting pro-mix within the heated chamber of such apparatus.

It is a still further object of the invention to provide a combustion system in which only main burner flame is monitored without an intermediate pilot.

Another object of the present invention is to provide a combustion system enabling a high degree of turn-down without affecting temperature uniformity.

Still another object of the invention is to provide a simple, low cost combustion system not requiring the use of refractory protective linings.

In accordance with the foregoing objects, there is provided a combustion system for uniformly heating a fluid in a chamber such as a stream of air passing through an air supply duct or a recirculating duct in a low temperature processing system or for providing heat to a liquid bath or chamber by direct contact of the products of combustion with the liquid to be heated. This combustion system utilizes a burner of the excess-air type, and the burner is mounted at a point external to the chamber so that the burner flame front is stabilized at a point external of the chamber. It is to be noted that terms such as duct, furnace, lehr or bath may be interchanged with the equivalent term chamber as the invention is generically applied to all these types of chambers. It should also be noted that the term excess-air has the usual connotation of air in excess of that required to stoichiometrically burn the fuel.

Attached to the aforesaid burner and extending into the aforesaid chamber is a ported flue product distribution tube adapted to distribute products of combustion from the burner in a predetermined pattern into the chamber to heat a passing stream of air or to heat a liquid within the chamber.

In the case of the application of the present invention to the heating of a fluid in a confined duct system such as fresh make-up air heaters or recirculating air heaters, the inherent disadvantages associated with the prior art use of line burners are eliminated. Products of combustion can be distributed within the air duct in any predetermined pattern by the sizing and spacing of the ports on the flue products distribution tube; thus upstream and downstream air disturbances may be compensated. Since there is no need to maintain a continuous curtain of flue gases along the length of distribution tube it is necessary to maintain a continuous flame along a line burner manifold to insure piloting from port to port), the dis tribution ports can be spaced widely apart making pos sible the placement of bearings etc. within the air duct as will be explained later. Further, bafiles may be used adjacent the distribution tube to direct the air to be heated past the ports and into confluence with the products of combustion to promote intimate mixing. The use of an excess air burner firing with excess air even at maximum capacity allows a greater temperature uniformity than line burners because of lower flame temperatures, and by excess air operation wherein gas is throttled and air flow is maintained constant, improved uniformity is experienced upon turn-down, contrary to prior art line burner operation.

The application of the ported tubeexcess-air burner combination to a submerged combustion heating system provides a means of utilizing an external burner with all its advantages without the attendant prior art disadvantage of having to use a refractory lining to protect the intermediate metal duct. This is possible by virtue of the cooler flame temperature afforded by excess-air operation.

Other and more specific objects of the invention will be apparent from the following specification and accompanying drawings, wherein like numbers are used throughout to identify like parts.

In the drawings:

FIG. 1 is a plan view, partially in section, of a make-up air supply system embodying the invention;

FIG. 2 is a sectional view taken along the line 22 of FIG. 1;

FIG. 3 is a sectional view taken along the line 3-3 of FIG. 2; and,

FIG. 4 is a fragmentary view showing a modified embodiment of the invention as applied to a make-up air supply system.

Referring now to the drawings and more particularly to FIG. 1, there is shown a make-up air supply unit 11 comprising an entrance section 12, a heater section 13, a re-expansion section 14- and a blower section 15. Arrows are used to indicate the flow of make-up air through various portions of the unit. Each of the sections 12, 13, 14 and 15 may be fabricated of sheet metal in any appropriate fashion and may be furnished with peripheral flanges 16 by which the sections can be connected to one another. The blower section 15, as shown, has two blowers l7 and it mounted therein. Each of the blowers 17 and 18 has axial air inlets 19 and a longitudinal air outlet 21). The blowers l7 and 13 are driven by a common shaft 21 which may be driven by means (not shown) located preferably on the outside of the unit 11. It is to be understood that the showing of two blowers in the blower section 15 is merely illustrative, and that any appropriate number of blowers may be used depend ing upon the capacity of each blower, size of the makeup air supply duct, etc. In practice, make-up air supply ducts are commonly supplied with from one to four lowers per duct.

Normally when two or more fan wheels are mounted on a common shaft it is necessary to install bearings between fans as well as external to the duct or heater sections. These bearings, not shown, would be mounted adjacent air inlets Ed on common shaft 21. It is obviously necessary to avoid any possibility of overheating these intermediate shaft bearings by high temperature flue product-air mixtures. When line burners are used, flue products may be distributed more or less uniformly across the duct, but since air flows are seldom uniform due to upstream disturbances such as bends etc., local high temperatures can still develop. This invention is particularly well suited to overcome this and similar problems because continuity of flame or line product discharge is not necessary; thus, the location and rate of line product discharge is extremely flexible, and problems such as overheating of intermediate shaft bearings can be easily avoided.

It is also to be noted that the blower section 15 may be located upstream from the burner section 13 instead of downstream as illustrated. Such an arrangement would enable the blowers l? and 13 to be operated at a cooler temperature, but this would involve the corresponding disadvantages of losing the effect of the fan as an additional mixing device in the intermixing of the streams of make-up air and flue products and increasing the required combustion air pressure because it would be necessary to operate against the fan static pressure.

The entrance section 12 is provided to minimize th effect of flow disturbances caused by inlet conditions. Air filters (not shown) may be provided either at the upstream end of section 12 or at the blower outlets depending on local requirements and duct system design.

As is shown in FIGS. 1 and 2, the heater section 13 is provided with a burner 22 mounted on an elongated flue product distribution tube 23 at a point external to the unit 11, and the tube 23 extends into and terminates within the section 13. The burner 22, as subsequently discussed in more detail and shown in PEG. 3, is of the nozzle mixing type, is fired on the excess air principle, and has a substantially cylindrical ignition and flame Stabilization chamber with inlet and exit ends. Fuel is introduced into the inlet end of the chamber in an axially flowing stream While a small percentage of the total combustion air is introduced tangentially to the chamber adjacent its inlet end to form a spinning stream of air encompassing the fuel stream. The burner has apertures adjacent the inlet end of the chamber suitable for insertion of piloting means and a flame monitoring device. Ignition of the fuel and air at the interface of the axially flowing fuel stream and the spinning air stream is effected by an electric or fuel torch introduced through one of the apertures, by spark ignition means, or by a fuel pilot inserted into one of the apertures. In case a pilot is used for ignition, it should be shut off after the main burner is operating. The balance of the combustion air is supplied to the chamber in a plurality of stages, downstream of the spinning air stream, and directed radially inward toward the axis of the chamber and the fuel stream.

Referring to FlG. 3, a burner 22 of the excess-air type, particularly adapted for use in a make-up air heater, is shown. The burner 22 comprises a body 24 which is made of fabricated or cast metal and is adapted to be attached to the tube 23. More particularly, a flange 25 on the burner body 2 is secured to a mating flange 2% on the tube 23 by any suitable means such as bolts (not shown) and packing 27 of a suitable material, such as sbestos, as interposed between the flanges 25 and 2d.

The burner body 2 has a back plate attached thereto by cap screws 29 extending through holes in the plate 23. An expanding throat section '75 is welded to a straight cylindrical wall portion 3b of back plate 255 to form an ignition and flame stabilization chamber 76 which is axially aligned with the tube 23).

A fuel supply pipe 31 is threaded into the back plate 23 and the end of the pipe 31 is flush with the inside face of the back plate 28. A tapped hole is also provided in the back plate 28 for receivin a spark plug 32 which is used to ignite the bur 22. Also a commercially available flame rod safety device (not shown) may extend through the back plate 28, and such a device or v1.

would be connected to a shut off valve in the line 31 should the flame in the ignition and flame stabilization chamber 75 of the burner 22 become extinguished.

Normally two or more inlet passages such as passage 33 (only one shown) are formed in the cylindrical wall section 36 tang nt to the internal wall of the chamber 76 and flush with the back plate 28. If two passages are used, the second passage would extend through the cylindrical wall section 30 at a point 180 from passage 33. Likewise, four equally spaced passages can be used, and in any case the passages are so formed so as to introduce the air so that one air stream augments the spinning action of the other or others.

A small percentage of the total combustion air, preferably 57%, is admitted through the tangential air inlet passages and this air enters chamber 76 tangentially to establish a spinning, slow mixing, stable flame front adjacent the back plate s8.

Additional air is admitted to chamber 7s by a plurality of radial air ports or apertures 34 in the expanding throat section 75 downstream from the tangential air inlet passages 33. The rate and degree of mixing of the entering air can be controlled by size, number and location of apertures 34, and these variables are normally so chosen so as to maintain combustion at a maximum rate without flame quenching. A convenient pattern for spacing the apertures 34 is in a plurality of axially spaced circumferential stages. Air entering each aperture 34 will produce a recirculating local eddy current which will promote stable burning with a maximum of mixing.

Attached to the burner body flange 2 5 is a cylindrical sleeve 77 which serves as an extension on the downstream end of expanding throat section 75. Sleeve 7'7 extends into the ported fine products distribution tube 23 and serves to shield the upstream end of tube 23 to avoid overheating of the tube which may occur at this end due to the insulation the exterior of tube 233 receives at the point where it passes through the wall of the air heating duct. A plurality of circumferentially arranged slots or apertures 78 are put into the sleeve 7'7 adjacent its upstream end. Slots 7 3 release a portion of the products of combustion to the annular space between sleeve 77 and tube 23 to serve the dual purpose of reducing the noise level of the burner and supplying combustion prod ucts to ports which may be located adjacent the inlet end of tube 23 entering radially inward into the aforesaid annular space.

The space between the burner body 24 and both the expanding throat section 75 and the cylindrical wall section 3% defines an annular air chamber to which air is admitted from air supply line 37 through orifice plate 79 and side inlet 35 of body 24. Orifice plate 79 is usually made with a plurality of small apertures rather than one large centrally located aperture to afford rapid pressure recovery. Orifice plate 79 is sized such that the air pressure drop across it will be approximately twice the pressure drop across the expanding throat section 75, i.e. twice the pressure drop through radial air ports 34. This makes the burner relatively insensitive to moderate changes in back pressure i.e. pressure changes which occur within the air heating duct. For example, a given change in back pressure will only change the pressure drop across radiant air ports 34 by /3 of the given change in the air heating duct and the change in air flow will be less than 10%. The air is properly distributed between the tangential inlet passages 33 and the radial apertures 34 by their relative areas.

As previously stated, the burner 22 is fired on the excess-air principle. For a more thorough disclosure of the construction and operation of such a burner attention is directed to copending application, Serial No. 808,161, filed April 22, 1959, now abandoned. The burner disclosed in such application is suitable for use with the tube 23 of the present invention, although it is preferably modified as herein described.

The burner 22 serves to burn streams of fuel from a fuel supply line 3% with air from an air supply line 37. The fuel supply line 38 is connected to the fuel inlet pipe 31, as shown in FIG. 1. The products of combustion from the burner 22 are discharged into the flue product distribution tube 23 from whence they pass into the make-up air stream through ports 39 in the tube 23.

Both the size and the relative location of the ports 39 are important factors in achieving the requisite degree of intermixing of the fine products and the passing make-up air stream. The degree of intermixing is the factor which determines the over-all degree of uniformity of the makeup air heating process, and size and location of ports determine mixing. In determining the size of the ports 39, the most important criterion is to limit port size to a value which will enable the pressure in tube 23 to be maintained at a value of approximately 0.5 to 2.0 inch w.c. (water column) higher than the pressure of the make-up air stream, i.e. the total area of all the ports 39 is such that the pressure drop across the ports, which act as orifices, is 0.5 to 2.0 inch w.c. When the tube pressure is significantly more than 2.0 inch w.c. higher than the air stream, the velocity of the flue products dis charged from the ports 39 will be excessive and may lead to quenching in flame separations in the make-up air stream. Tube pressures substantially less than 0.5 inch w.c. higher than that of the air stream are insufficient to cause an even discharge of flue products from the various ports, a factor which may also give rise to problems of temperature uniformity.

In determining the relative location of the ports, two approaches may be taken as are illustrated in FIGS. 1 and 4. The embodiment of FIG. 1 differs from the embodiment of FIG. 4 primarily in that the former uses internal baffling means 4b to promote the intermixing of the makeup air and the fine products from the distribution tube 23.

In FIGS. 1 and 2 the bafiiing means 40 is shown as comprising vertically extending baffle plates 41, 42; and 43 which form, respectively, vertical air passages 44 and 45, it being noted that there is one such vertical passage for each blower 17 and 18. In the embodiment illustrated in FIG. 1, the ports 3% are so located as to discharge flue products evenly at points adjacent each vertical air passage, it being further noted that the ports 39 are located so as to discharge the flue products perpendicularly into the passing air stream of make-up air. To achieve optimum temperature uniformity, the ports 39 are located within the passages 44 and 45 at points slightly offset from the centers of the fans 17 and 18 respectively.

An improved degree of intermixing is obtainable in the baflied arrangement of FIG. 1, as compared to a nonbafiled arrangement of the type shown in FIG. 4 which will be described later in detail. This improved intermixing is believed to be the result of the turbulence of re-expansion which occurs when the rapid air streams which emerge from the narrow passages 44 and 45 are suddenly allowed to re-expand to fill a much larger volume. To obtain the full efiect of such a re-expansion it is necessary to provide a sufficiently long passage in which substantially complete re-expansion can occur. It is for this reason that re-expansion section 14 has been provided between the blower section 15 and the heater section 13. Also, baffling means 4t) serve to screen out variations in upstream air velocities.

The alternative embodiment illustrated in FIG. 4 comprises a heater section 113 located intermediate the entrance section 12 and the blower section 15. The heater section 113 includes the burner 22 mounted outside thereof which fires into a flue products distribution tube 123. A number of ports 139 are located along the top side of the tube 123, it being noted that there is a corresponding row of ports (not shown) located along the under side of the tube 123. The size of ports 139 is determined in the same manner as is the size of the ports 39 in the tube 23, i.e. the total area of all ports is limited to provide a port pressure drop of approximately 0.5 to 2.0 inch we. The number and spacing of the ports 139 is largely a matter of design selection, it being noted that the greater the number of ports the better the degree of intermixing of flue products and air that can be obtained. Again attention is directed to the fact the ports can be located at any desired position as contrasted to line burners in which port position is relatively inflexible because flame continuity is required.

It has been previously noted that the baflled heater arrangement of FIG. 1 is capable of more uniformly heating the malre-up air stream than is the unbaifled heater arrangement of HG. 4. In this connection it should be further noted that the arrangement of FIG. 4, which is somewhat less expensive than the arrangement of FIG. 1, is nonetheless capable of achieving the degree of make-up air heating uniformity required in the majority of installations.

A flue products distribution tube of the type shown in PEG. 4 may also be used to heat a liquid in a chamber. More particularly, the tube is inserted into the liquid either through the side of the container or preferably from the top surface of the liquid, and a burner, such as the burner 22 of FIGS. 1 to 4, of the excess-air type is mounted on the tube.

The heated flue products leave through ports into the liquid thereby heating the same, and these ports preferably have a diameter of approximately one inch. It has been found that the tube should be submerged between 12 and 18 inches below the top surface of the liquid for the maximum heating and that when the tube is so positioned, substantially 100% recovery of the heat input is realized. However, no additional benefits are obtained by submerging the tube to a depth greater than 18 inches, and a greater submersion would be undesirable because of the increased head on the ports. Inasmuch as the pressure within the tube must be greater than the pressure of the liquid in order that the flue products will leave the ports and heat the liquid, it has also been found that the tube will not heat the liquid if it is submerged less than four inches from the top surface of the liquid because the flue products merely blow the liquid to one side without heating it.

The present invention can also be used in a combination submerged combustion-makeup air heating system. This is particularly advantageous where the humidity as well as the temperature of the make-up air must be controlled such as in applications where the make-up air is supplied to paint spray booths and a high, constant humidity is required to obtain product uniformity. In this case the flue products distribution tube would be located in a water sump, and the water heated therein would be sprayed Within the makeup air heating duct to heat and humidify the air passing through. The firing of the excess-air burner can be controlled from a temperature or humidity signal or a combination thereof.

As has been previously noted, the burner unit 22 is of the excess-air type, that is, it is a burner which can be readily turned down to low capacity simply by modulating the fuel supply While maintaining the supply of combustion air to the burner at a constant value. The features and advantages of excess-air combustion as compared to other combustion techniques are extensively discussed in US. Patent 2,952,307 to Schramm et al.

The application of the excess-air combustion principle to makeup air heating equipment in accordance with this invention is of special importance because it enables the pressure within the ported distribution tubes 39 and 139 to be maintained at a practically constant value over the entire range of burner firing rates. Mass air flow remains virtually constant over the entire turn-down range so that relative mixing performance remains constant. Since flame temperature decreases with turn-down, temperature uniformity increases rather than decreases as when using line burners.

Excess-air operation of the burner 22 may be achieved in the following manner as is illustrated in the schematic piping arrangement of burner 22 in FIG. 1; a substantially constant rate of air is delivered from a blower 46 to the burner 22 through the air supply line 37 which includes a shut-off valve 47 and a metering orifice 48. A variable quantity of fuel is delivered to the burner 22 from a fuel source (not shown) by the fuel supply line 38, which includes a pressure regulating valve 49, a fuel metering orifice 5t) and a thermostatic modulating valve 51 adapted to modulate the flow of fuel to the burner 22 as a function of heating demand determined by a thermostat 52 located as shown in the inlet section 12, or located in re-expansion section 15. Thermostat 52 may also be located outside of the make-up air supply system and in the enclosure being heated.

An important advance over the prior art by the present invention is the provision of a positive remedy to one of the most severe of all safeguard problems in low temperature fluid heating. In normal practice, separate pilot ignition and flame stabilization is used with the flame protection device monitoring both pilot and main flame. If the main mixture stream cannot be ignited because of an air-fuel ratio change or other reason, the flame monitoring device being heated by the pilot will continue to indicate satisfactory combustion conditions, and an explosion may result. With the present invention, since use of a separate pilot is eliminated, the flame protection device monitors the main burner input over the entire turndown range.

While the preferred embodiments of the invention have been disclosed, various modifications can be made to the tube and excess air burner without departing from the spirit of the invention or the scope of the subjoined claims.

I claim:

1. In combination with a duct, apparatus for heating a stream of fluid flowing through the duct comprising, in combination: a burner adapted to promote the combustible reaction of streams of fuel and combustion supporting air, the ignition and flame stabilization portion of the burner being located external to the duct; a tube adapted to receive the products of combustion from the burner, said tube extending into the duct; a plurality of ports along the tube whereby products of combustion from the burner are discharged from said ports into the fluid flowing through the duct; and a sleeve axially aligned with the burner and having its forward end attached to the flame stabilizing portion thereof, said sleeve extending into said tube and having apertures adjacent the forward end of the sleeve for release of combustion products therethrough into the annulus defined by the tube and the sleeve.

2. In combination with a duct, apparatus for heating a stream of fluid flowing through the duct comprising, in combination: a burner adapted to promote the combustible reaction of streams of fuel and combustion supporting air, the ignition and flame stabilization portion of the burner being located external to the duct; a tube adapted to receive the products of combustion from the burner, said tube extending into the duct; a plurality of noncontiguously disposed ports along the tube whereby prod nets of combustion from the burner are discharged from said ports into the fluid flowing through the duct; and further comprising baffle means within said duct adjacent said tube, said baflle means being adapted to divide said stream of fluid into a plurality of individual streams.

3. Apparatus according to claim 2 wherein said ports are disposed along the tube in the path of said stream of air formed by said baffle means.

4. A method of heating a stream of fluid flowing through a duct comprising the steps of: stabilizing a combustible reaction between streams of fuel and combustion air at a point external to the duct, passing the products from the combustible reaction into the duct in an enclosed stream, discharging the products from the enclosed stream into the stream of fluid flowing through the duct in a plurality of non-contiguous branch streams, and baffling the stream of fluid to divide the stream of fluid into a plurality of streams and to direct each of the plurality of streams into contact with one of the noncontiguous branch streams.

5. Apparatus for heating a stream of fluid flowing through a duct comprising, in combination: a duct; blower means for circulating fluid through said duct; a burner having an ignition and flame stabilization portion adapted to promote the combustible reaction of streams of fuel and combustion supporting air, the ignition and flame stabilization portion of the burner being located external to said duct; combustion air supply means for delivering a stream of combustion supporting air to said burner at a substantially constant rate; fuel supply means for delivering a stream of fuel to said burner; control means responsive to the temperature in the interior of the duct for reducing the flow of fuel only to said burner during periods of reduced heat demand; a tube adapted to receive the products of combustion from said burner, said tube extending into said duct; and a plurality of ports along said tube whereby products of combustion from said burner may be discharged from said tube through said ports into said fluid stream flowing through said duct.

References Cited in the file of this patent UNITED STATES PATENTS 1,502,200 HoWlett et al. July 22, 1924 2,188,528 Clark Jan. 30, 1940 2,227,666 Noack Jan. 7, 1941 2,402,803 Chandler June 25, 1946 2,584,606 Merriam et al Feb. 5, 1952 2,628,087 Mayer Feb. 10, 1953 2,836,409 Harrison May 27, 1958 2,840,362 Krieble et a1 June 24, 1958 

4. A METHOD OF HEATING A STREAM OF FLUID FLOWING THROUGH A DUCT COMPRISING THE STEPS OF: STABILIZING A COMBUSTIBLE REACTION BETWEEN STREAMS OF FUEL AND COMBUSTION AIR AT A POINT EXTERNAL TO THE DUCT, PASSING THE PRODUCTS FROM THE COMBUSTIBLE REACTION INTO THE DUCT IN AN ENCLOSED STREAM, DISCHARGING THE PRODUCTS FROM THE ENCLOSED STREAM INTO THE STREAM OF FLUID FLOWING THROUGH THE DUCT IN A PLURALITY OF NON-CONTIGUOUS BRANCH STREAMS, THE PLURALITY OF STREAMS INTO CONTACT WITH ONE OF THE NONCONTIGUOUS BRANCH STREAMS. 